Anthropogenic actions are altering fluxes of nitrogen (N) in the biosphere at unprecedented
rates. Efforts to study these impacts have concentrated in the Northern hemisphere, where
experimental data are available. In tropical developing countries, however, experimental studies are
lacking. This paper summarizes available data and assesses the impacts of human activities on N
fluxes in Puerto Rico, a densely populated Caribbean island that has experienced drastic landscape
transformations over the last century associated with rapid socioeconomic changes. N yield calculations
conducted in several watersheds of different anthropogenic influences revealed that disturbed
watersheds export more N per unit area than undisturbed forested watersheds. Export of N
from urban watersheds ranged from 4.8 kg ha)1 year)1 in the Rı´o Bayamo´ n watershed to
32.9 kg ha)1 year)1 in the highly urbanized Rı´o Piedras watershed and 33.3 kg ha)1 year)1 in the
rural-agricultural Rı´o Grande de An˜ asco watershed. Along with land use, mean annual runoff
explained most of the variance in fluvial N yield. Wastewater generated in the San Juan Metropolitan
Area receives primary treatment before it is discharged into the Atlantic Ocean. These
discharges are N-rich and export large amounts of N to the ocean at a rate of about
140 kg ha)1 year)1. Data on wet deposition of inorganic N (NHþ4
þ NO
3 ) suggest that rates of
atmospheric N deposition are increasing in the pristine forests of Puerto Rico. Stationary and
mobile sources of NOx (NO+NO2) and N2O generated in the large urban centers may be
responsible for this trend. Comprehensive measurements are required in Puerto Rico to quantitatively
characterize the local N cycle. More research is required to assess rates of atmospheric N
deposition, N fixation in natural and human-dominated landscapes, N-balance associated with
food and feed trade, and denitrification.